1. Field of the Invention
The present invention relates to pilot valves of the type used to send an output signal pressure to a pneumatically operated process valve for controlling the valve and, specifically, to an improved flapper actuated pilot valve were the output signal pressure is proportional to the force supplied by the flapper.
2. Description of the Prior Art
The oil and gas, chemical and other industries utilize process valves for controlling the flow of process fluids are operated by means of a pneumatic control signal. The pneumatic control for such valves typically includes a pilot valve whose function is to send an output signal pressure to the pneumatic controller, which either opens or closes the process valve.
For instance, in the case of an oil and gas separator tank, a liquid level controller is provided which uses a "float" or displacement type sensor to transmit changes in the liquid level in the tank to a pilot valve outside the vessel. The pilot valve signals a process control or discharge valve in the discharge outlet from the vessel to open or close the discharge valve in response to the liquid level within the vessel. The float within the vessel interior is connected to a "flapper" outside the vessel which, in turn, acts upon the pilot valve.
Supply gas pressure is generally taken from the production gas and routed to the pilot valve. When the liquid level in the vessel is within the desired limit, the supply gas is vented through the pilot valve to the atmosphere. When the liquid level rises sufficiently to change the position of the float, the flapper applies a force to the pilot valve so that supply gas is diverted within the pilot valve to thereby provide a control signal to the discharge valve which allows liquid to flow from the vessel.
U.S. Pat. No. 4,512,365, filed Dec. 8, 1983, entitled "Pilot Valve For An Oil And Gas Separator", assigned to the assignee of the present invention, shows such a pilot valve in which the output control pressure is not dependent upon the force supplied by the flapper to the pilot valve. That is, the pilot valve operates in "on-off" fashion to either emit an output control pressure or to vent the control pressure to the atmosphere. In other types of pneumatically controlled valves, such as those featuring Bourdon tube controllers, it is desirable to provide a pilot valve which outputs a control pressure which is dependent upon the flapper force applied to the pilot valve.
U.S. Pat. No. 4,699,175, filed Oct. 27, 1986, entitled "Flapper Actuated Pilot Valve", also assigned to the assignee of the present invention, was an improvement to the earlier design in providing a flapper actuated pilot valve in which the output control signal emitted by the valve was proportional to the flapper force applied to the valve. The present invention is directed to further improvements in pilot valves of the type which output a control pressure which is dependent upon the flapper force applied to the pilot valve.